Bluetooth is a wireless technology standard for exchanging data over short distances (using short-wavelength UHF radio waves in the ISM band) from fixed and mobile devices and building personal area networks (PANS). Bluetooth operates at frequencies between 2400 and 2483.5 MHz (including guard bands 2 MHz wide at the bottom end and 3.5 MHz wide at the top). This is in the globally unlicensed Industrial, Scientific and Medical (ISM) 2.4 GHz short-range radio frequency band. Bluetooth uses a radio technology called frequency-hopping spread spectrum. Bluetooth divides transmitted data into packets, and transmits each packet on one of 79 designated Bluetooth channels. Each channel has a bandwidth of one MHz. Bluetooth 4.0 uses 2 MHz spacing, which accommodates 40 channels. The first channel starts at 2402 MHz and continues up to 2480 MHz in 2 MHz steps. It usually performs 1600 hops per second, with Adaptive Frequency-Hopping (AFH) enabled.
Bluetooth low energy (Bluetooth LE, BLE) is a wireless personal area network technology designed and marked by the Bluetooth Special Interest Group aimed at novel applications in the healthcare, fitness, beacons, security, and home entertainment industries. Compared to Bluetooth, BLE is intended to provide considerably reduced power consumption and cost while maintaining a similar communication range. BLE uses frequency hopping to counteract narrowband interference problems. The LE system employs a frequency hopping transceiver to combat interference and fading and provides many frequency hopping spread spectrum (FHSS) carriers. FHSS is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels.
The majority of channel signal to noise ratios (SNRs) vary within +/−5 dB. Without fast rate adaptation, the radio needs to operate at the lowest data rate or high SNR to maintain the PER. More than two data rates might be required for 10 dB SNR variations. In Bluetooth/BLE, Channel Quality Driven Data Rate Change (CQDDR) is a channel rate control algorithm implemented in Link management protocol (LMP) for rate adaptation. The signaling exchange of LMP messages relies on the underlying physical layer link, which might not work well in poor signal-to-noise ratio condition.
While short-range Bluetooth/BLE communication is between individual radios within a distance of 5 to 10 meters, long-range Bluetooth/BLE calls for a distance of about 100 meters or longer. The long-range channel characteristics is considerably different from the short-range channel characteristics. Long-range channels have lower operating SNR, frequency selective fading, and has faster time domain variations along the propagation path. The hopping channels may have significantly different SNRs because it is difficult to find hopping sequence/channel map satisfying all nodes within a Piconet. Longer packet length is also more susceptible to mid-packet collisions and requires trickier link management operation. If the link management is not robust, then long-range communication also results in higher power consumption due to re-transmission.
The existing LMP/CQDDR is a MAC layer protocol. The TX and RX radios use the PHY layer PDU to transmit LMP messages and need to establish handshake. Overall, LMP is a very slow adaptation process: the receiver MAC layer operation detects channel degradation usually after multiple packets, and then requests a preferred rate to the transmitter. If the physical link is nearly lost, or the ACK is not properly received, the radio can make a few retries until the LMP message is successfully acknowledged and the transmitter can switch to preferred rate after negotiation. If link is lost, both sides rely on timeout to try different PHY rates. In some cases, the PHY layer link might be broken and LMP messages cannot be reliably exchanged. Another drawback is that transmitter cannot unilaterally switch PHY data rate without message exchange with the receiver or after time-out. As a result, the existing LMP/CQDDR is difficult to adapt properly with varying SNRs at different channels, especially for long range Bluetooth or BLE communication. Further, the transmitter that purely relies on LMP message for link adaptation would not easily attempt high data rate since, if link is not reliably at higher rate, it would take a long time to recover.
A solution is sought to improve rate adaptation for long-range Bluetooth/BLE communication.